Even when they look clean, tanks can be dangerous; for instance, the presence of water inside might produce corrosion, which will decrease the level of oxygen and produce hydrogen, which is a flammable gas. They might, as well, contain traces of the substance previously used in the tank, or, if they have been cleaned, they might contain the inertizing substance. In conclusion, the safety of the employees can be guaranteed only by using a multigas detector.

GASES INVOLVED

Oxygen (O2): the general principle in order to inertize a tank is to depurate the atmosphere with a gas that does not contain oxygen, preventing a possible combustion. It must be remembered that both during and after the depuration process, the atmosphere inside the tank will not contain oxygen.
Flammable gases: the combustion of flammable vapour inside the tank and the subsequent explosion can have a considerably violent reaction.
Sulphured hydrogen (H2S): usually due to the product previously contained in the tank. For instance: oil-based materials, which can produce not only flammable gases but also toxic sulphured hydrogen.
Nitrogen dioxide (NO2): usually due to the product previously contained in the tank.

EXPECTED RISKS

O2: both excess and lack of oxygen may cause important diseases, sometimes death.
Inflammable gases: in smaller concentration even than LEL with regard to the volume they explode in presence of a minimum flash source often resulting in serious consequences for people and things.
H2S: colourless gas stands out for its characteristic smell of rotten eggs. A low-concentration exposure causes eye and throat irritation, cough, acceleration of breathing and fluid formation in the respiratory ways. High concentrations kill the olfactory nerve so making impossible to smell its disgusting odour, and may cause unconsciousness in few minutes.
NO2: nitrogen dioxide irritates lung ways seriously. Even at moderate concentration in air, it causes acute cough, pains at the chest, convulsions and blood circulation failure. Also it may cause irreversible damages to lungs which may reveal themselves even many months after.